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Gymnosperm
'''Gymnosperms' (known sometimes as Gymnospermae) are seed-bearing plants whose seeds do not form inside fruits, but outside of the ovum. Gymnosperms, once an authoritative and major classification level in its kingdom, are now a rather heterogenous group of seed-bearing plants which include the conifers, cycads, ginkgos, and gnetophytes. The word, "gymnosperm" is derived from the Greek word, ''gymnospermos'' (γυμνόσπερμος), meaning "naked seeds", due to the unenclosed condition of their seeds. Their naked condition contrasts with the seeds or ovules of the angiosperms, which are enclosed during pollination. The seeds of gymnosperms develop either on the surface of scale or leaf-like appendages of cones, with the exception of Ginkgo, where the seeds are developed on the end of short stalks. The gymnosperms and angiosperms form the spermatophytes (seed plants). The largest group of living gymnosperms are the conifers, followed by the cycads, then the Gnetophyta (Gnetum, Ephedra, and Welwitschia), and finally Ginkgo, with only one living species. Classification :: For more information, see Spermatophyte ]] In earlier classification, the gymnosperms were regarded as a "natural" group. There is conflicting evidence on the question of whether living angiosperms form a clade. The fossil record of gymnosperms includes many distinct taxa that do not belong to modern groups, such as the Pteridophyta, which are seed-bearing trees that have a somewhat fern-like vegetative morphology. When fossil gymnosperms, such as Bennettitales, ''Caytonia, and the glossopterids are considered, it is clear that the angiosperms are nested within a larger gymnosperm clade, although the group of gymnosperms that are their closest relative remains unclear. Diversity and origin '']] There are between 700 and 900 species of gymnosperms currently living in this time period. It is widely accepted that the gymnosperms originated during the Carboniferous. Early characteristics of seed plants were evident in the fossils of Progymnospermophyta from the late Devonian around 380 million years ago. It has been suggested that pollination of some extinct groups of gymnosperms were caused by extinct species of scorpionflies during the Mesozoic. The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar coevolution of nectar-feeding insects and angiosperms. Conifers are by far the most abundant extant group of angiosperms, with about 600-630 species in 65-70 genera in 6-8 families. Conifers are woody, and most are evergreen. The leaves of most conifers are long, thin, and resemble a needle, but others, such as the Cupressaceae and Podocarpaceae have flat, triangular leaves. Cycads are the next most abundant group, containing about 130 species. The Gnetales follow the cycads, with about 70-80 species, and the ginkgos follow the Gnetales, with only one extant species. Uses Gymnosperms provide several major economic uses. Pines, fir, spruce, and cedar are all used for their lumber. Some other common uses of gymnosperms are in soaps, varnish, paint, food, and some perfumes. Life cycle Gymnosperms are spore-bearing plants (Sporophytes), with a sporophyte-dominant lifestyle, like in all other vascular plants the gametophyte is relatively short-lived. Two spore types, microspores and megaspores, are usually created in pollen cones or ovulate cones. A short-lived multicellular haploid gametophyte develops inside the spore wall. Pollen grains (microgametophytes) mature from microspores, and produce sperm cells, while the megagametophyte tissue is developed in the megaspore of each ovule, and produces egg cells. Thus, megaspores are enclosed in ovules and give rise to the megagametophytes, and eventually egg cells. During pollination, pollen grains are physically transferred between plants, from the pollen cone to the ovule, by wind and insects. Whole grains will enter each ovule through a microscopic gap in the integument called the micropyle. The pollen grains mature further inside the ovule, and eventually produce sperm cells. Two main methods of fertilization are found in gymnosperms. Cycads and ginkgos have motile sperm that can swim directly to the egg inside the ovule, while conifers and gnetophytes have sperm without a flagellum, and are conveyed to the egg along a pollen tube. After fertilization, the zygote develops into an embryo. More than one embryo is usually initiated in each gymnosperm seed. Competition between the embryos for needed resources within polyembronic seeds produces programmed cell death to all but one of the embryos. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the integument. External links * Category:Gymnosperms Category:Plantae Category:Plants Category:Taxonomy